Process for the production of aluminum-steel composite

ABSTRACT

A HIGH STRENGTH ALUMINUM-STEEL COMPOSITE IS FORMED BY COMPRESSING AT ELEVATED TEMPERATURES AN ASSEMBLED STACK OF AT LEAST TWO SHEETS OF ALUMINUM HAVING STEEL WIRES SANDWICHED THEREBETWEEN, WITH ONE OF THE OPPOSING SURFACES OF THE SHEETS HAVING A GROOVE FORMED THEREIN, THE FORCE UNDER WHICH THE ASSEMBLY IS COMPRESSED BEING SUFFICIENT TO FILL IN THE GROOVE AND THEREBY EXPOSE FRESH METAL SURFACE WHICH FORMS A SUBSTANTIALLY CONTINUOUS METALLURGICAL BOND BETWEEN THE OPPOSING SHEET SURFACES.

1971 E. v. SUMNER ETAL 3,551,996

PROCESS FOR THE PRODUCTION OF ALUMINUM-STEEL COMPOSITE Original FiledOct. 2, 1967 United States Patent 3,551,996 PROCESS FOR THE PRODUCTIONOF ALUMINUM-STEEL COMPOSITE Edwin V. Sumner, Alhambra, Donald Q. Cole,Los

Angeles, and Le Roy W. Davis, San Pedro, Calif., assignors to HarveyAluminum, Incorporated Application Oct. 2, 1967, Ser. No. 672,048, whichis a continuation-in-part of applications Ser. No. 571,301 and Ser. No.571,322, both filed Aug. 9, 1966. Divided and this application Aug. 14,1969, Ser. No. 862,132 Int. "Cl. B23k 31/02 U.S. Cl. 29-4723 9 ClaimsABSTRACT OF THE DISCLOSURE A high strength aluminum-steel composite isformed by compressing at elevated temperatures an assembled stack of atleast two sheets of aluminum having steel wires sandwiched therebetween,with one of the opposing surfaces of the sheets having a groove formedtherein, the force under which the assembly is compressed beingsufiicient to fill in the groove and thereby expose fresh metal surfacewhich forms a substantially continuous metallurgical bond between theopposing sheet surfaces.

This is a divisional application of co-pending United States patentapplication Ser. No. 672,048, filed Oct. 2, 1967, which is acontinuation-in-part application of our co-pending United States patentapplications, Ser. Nos. 571,322 and 571,301 each filed Aug. 9, 1966.

In the conventional manufacture of aluminum-steel composites, steel wireis placed between aluminum sheets in a groove formed in the surfacethereof the resultant stacked sheets then being passed between a pair ofrolls to force aluminum metal into the groove and about the surface ofthe wire (see U.S. Patent No. 3,201,862). When high strength steel wireis consolidated according to this process, only a relatively low rollingreduction of the stacked sheets can be effected without rupture of thewire because aluminum being extruded outwardly may stress the brittlewire in contact therewith beyond its elastic limit.

By carrying out this process with only a small rolling reduction in thesheet thickness, the oxide coating on the surface of the sheet remainssubstantially intact and little or no fresh aluminum surface is exposedto form a sound metallurgical bond. Consequently, the resultant rolledsheets are held together at their surfaces principally by means of weakmechanical bonds formed between the aluminum oxide coatings and usuallyless than about 15% of the sheet surface is bonded metallurgically to anadjacent sheet. Since rupture of the mechanical bonds and resultantdelamination of individual sheets can occur under conditions of highstress, composites produced in this manner are unsatisfactory for manyapplications.

Efforts have been made to produce a stronger aluminum-steel compositeand minimize weak mechanical bonds in its matrix by flame sprayingaluminum in an inert atmosphere onto a plurality of spaced steel wires.The sprayed aluminum matrix is, however, relatively weak due to its highporosity and the resultant composite is unsuitable in many applicationswhere high matrix strength is needed. Although the strength of thesprayed matrix can be increased by rolling the composite to reduce itsporosity, loading of the steel wire in tension during the rollingoperation often causes rupture of at least a portion of the wires andtherefore the gain in matrix strength is offset by the loss in strengthdue to wire rupture.

In another process for the manufacture of aluminumsteel composites, amesh of woven stainless steel Wires "ice and soft aluminum wiressandwiched between aluminum sheets is hot rolled and then solution heattreated to merge the aluminum wires into the aluminum sheet (see U.S.Patent No. 3,314,825). The sheets forming this composite are heldtogether, for the most part, by mechanical bonds, the only metallurgicalbonds between sheets occurring at the common juncture of the embeddedaluminum wire with adjacent sheets. Without the benefit of a continuousand uninterrupted metallurgical bond between the sheets over theirentire surface, delamination of these composites can occur in serviceconditions of high stress.

It is therefore a principal object of the invention to provide animproved aluminum steel composite suitable for use in high stressapplications.

Another object is to provide an improved aluminumsteel composite withsteel wires embedded in a homogeneous aluminum matrix.

Yet another object is to provide in the consolidation of the steel wirean improved process of forming a sound metallurgical bond betweenopposing surfaces of the aluminum sheets.

Still another object is to provide in the production of analuminum-steel composite an improved hot rolling process which effectsconsolidation of the wire and aluminum sheets without danger of wirerupture.

These and other objects and advantages will become apparent uponreference to the following description, drawings and claims appendedhereto.

It has been surprisingly found that a high strength aluminum-steelcomposite having a homogeneous aluminum matrix can be formed byconsolidating steel wire between aluminum sheets, with one of theopposed sheet surfaces having at least one elongated groove thereinextending transverse to the direction of the wire. By compressing suchan assembly of sheets at an elevated temperature, it was unexpectedlyfound that the sheets are held together over their surfaces by asubstantially continuous metallurgical bond.

The aluminum sheets employed herein preferably have a plurality ofsubstantially parallel grooves or recesses formed in their surface, itbeing advantageous to space the grooves uniformly over the entire sheetsurface. A sufficient number of such grooves are preferably formed inthe sheet to increase the surface area thereof from about 5 to 300, morepreferably from about to 200 percent.

After cleaning the surface of sheets to be compacted, the grooves can beformed therein by any of the conventional machining operations. Whererelatively thin sheet aluminum is to be compacted, such as aluminumfoil, wire brushing of the sheet surface can be used to increase thearea thereof to from about 2 to 3 fold. Formation of the grooves in thesheet surface during continuous operations can also be achieved, forexample, by passing the sheet through a pair of embossing rolls.

By compressing an assembly of such grooved sheets beyond the elasticlimit of the aluminum, the surface of the sheet distorts and metal flowsinto the grooves. Upon disrupting the aluminum oxide film on the sheetsurface, fresh unoxidized aluminum is exposed which can, upon contactwith the surface of the adjacent sheet, metallurgically bond thereto. Toinsure the formation of a continuous metallurgical bond over the entiresheet surface, the grooves or other similar deformations in the sheetare preferably spaced uniformly over the surface so that at least somemetal movement at or near the sheet surface will occur duringcompaction. Although there is no limit to the number of grooves whichcan be used, it is preferred to employ sheets having a suflicient numberof grooves to effect complete disruption of the oxide film duringcompaction.

During compaction, the freshly exposed aluminumox:

idizes almost immediately and the resultant oxide film acts as a barrierto inhibit the formation of a metallurgical bond. To prevent oxidationof the freshly exposed alumi. num surface before it-can be bonded toanother surface, it is preferred to carry out the compaction operationeither under a vacuum of less than 100 mm. Hg, or else, in an inertatmosphere such as nitrogen or argon.

In rolling a stacked assembly of such sheets, the volume of the assemblyis reduced an amount corresponding to the controlled void volume betweenthe sheets created by the grooves or recesses in the sheet surface.During this rolling operation, it is preferred to first' reduce thethickness of the assembly until all voids are eliminated and then rollthe resultant compact an additional amount to reduce its thickness up toabout percent. Compaction to roll out the controlled void volume can beeffected between a first pair of hot rolls with the additional reduc;tion in thickness of the resultant composite ,being optionally achievedby cold rolling'Where such a double rolling operation is employed, it ispreferred that the sub sequent cold rolling reduce the compositethickness no more than about 2 percent.

In the initial compaction operation where the controlled void volume isrolled out of an assembly, the sheets are preferably maintained attemperatures of from about 400 to 1100, more preferably from about 700to 900 F. for periods ranging from about 10 minutes to 10 hours,preferably for from about 30 to 120 minutes. Although compactiontemperatures lower than 400 F. can be used with certain aluminum alloys,superior metallurgical bonding of the sheet is obtained in most casesusing temperatures of at least 400 F. As far as the aluminum-alminumbonding is concerned, elevated temperatures even approaching the meltingpoint of the aluminum are satisfactory. At temperatures above 1100 F.,however, it has been found that filaments of most steel alloys aredeleteriously affected and soften or become annealed.

The duration of the compaction and heating is important in effecting astrong metallurgical bond, it being preferred to maintain the compositeat the elevated temperature until diffusion of aluminum from one sheetto another across the interface therebetween has occurred. Beforecommencing production on a commercial scale with any particular aluminumand steel alloys, a series of routine tests can be easily conducted todetermine the optimum elevated temperature and period of heating for anycombination of aluminum and steel alloys.

It has also been found that brittle intermetallic compounds of aluminumand steel form during compaction on the surface of the filaments attemperatures in excess of about 1100 F. It is therefore desirable inconducting routine testing for the determination of the optimumtemperatures to inspect the filament surface for the presence of suchbrittle intermetallic compounds. The steel wire to be consolidated isplaced on the sheet surface at an angle transverse to the longitudinalgroove or grooves therein. In applications where high tensile strengthis required, a plurality of steel filaments can be used between eachpair of sheets to produce a composite having the desired tensilestrength. Generally, the steel in the composite constitutes from about 1to 60, preferably from about 10 to 40, more preferably from about 22 to28 percent by volume. The composites having a low steel content areparticularly suitable for use as cladding on atomic reactorfuel .cells.A high steel content composite can be advantageously used in structuralapplications in which a high strength to weight ratio in one directionis needed. To provide afresh clean surface free of excess oxid film canimpair bonding, it is preferred to throughly clean the surface of thealuminum sheet before compaction. A suitable cleaning procedure can becarried out, for ex ample, by first treating in an alkaline bath havinga pH of from about 9 to 11, water rinsing, acid rinsing, followed by awarm water rinse. The resultant cleaned sheet canthen .be allowed tostand .andairdry at ambient .tem-

perature. Where the sheet is to be preheated before compaction, the wetsheet from the cleaning baths can be passed directly to a preheater.

Consolidation of the steel wire between sheets of aluminum can beadvantageously effected, for example, in a conventional 200 tonhydraulic press having electrically heated dies.-Continuous productionis preferably carried out by feeding the sheet aluminum and sheet wirethrough the nipof a pair of oppositely, rotating, heated, spacedrollers. i

A preferred embodiment showing the composite of the present inventionand process by which theyare made is illustrated further in theaccompanying drawings, in which:

FIG. 1 is a schematic view in partial section of an arrangement forproducing a composite of aluminum and steel according to the presentinvention;

FIG. 2 is a side elevational view in cross section of an assembly ofaluminum sheets with steel wire sandwiched therebetween, illustratingparticularly the grooves formed in the surface of the sheet;

FIG. 3 is aside elevational'view taken along line 33 of FIG. 1,illustrating a composite produced according to the present invention;and

FIG. 4 is a side elevational view of a composite of the presentinvention having several rows of steel wire in an aluminum matrix.

With reference to the drawings, a hot rolling mill shown generally at 1(FIG. 1) comprises a housing 3 completely encasing the mill, a pair offeed rolls 5 on which is wound aluminum sheet 6, feed roll 7 on whichseveral reels of steel wire or strip can be stored, a pair of opposed,OPT p ositely rotatable, hot rolls 11 and a composite take-up roll 15.Before operation is begun, the housing 3 surrounding the hot milliseither evacuated, or else, charged with an atmosphere of an inert gasas described hereinbefore. As shown, a plurality of steel wires orstrips 8 are continuously removed from feed roll 7 and then sandwichedbetween aluminum sheets 6 to form an assembly suitable for compaction(FIG. 1).

According to the present invention, the steel wire is brought intocontact with surface 17 of sheet 6 having at least one groove or recessformed in its surface. The wire in the assembly extends transverse tothe groove or grooves-in the sheet surface, it being preferred that thewire be placed at an angle normal to the direction of the groove. Theassembly is then introduced between hot rolls 11 where it is heated andcompressed to move aluminum into the grooves and firmly consolidate thesteel wires into the aluminum sheet. The resultant composite 23 (FIGS. 1and 3) comprises a homogeneous aluminum matrix 27 with steel wiresmechanically bonded to and encased therein. If' desired, emergingcomposite 23 can then be taken up on roll 15, or else,'cut into segmentsand stacked.

In the arrangement shown in FIG. 1, the aluminum sheet'6 on rolls '5 hasbeen properly grooved, cleaned and dried before being rolled. Althoughthe grooves 19 in sheets 6 are round, grooves of any cross-sectionalarea can be employed. It can also be seen that sheet 6 are round,grooves of any cross-sectional area can be employed. It can also be seenthat sheet 6 is introduced between rolls 11 with the grooves 19-lyingsubstantially parallel to the roller axis, it also being preferred toroll the assembly in the direction of the filaments.

In FIG. 4 several rows of steel wire are shown consolidated into ahomogeneous aluminum matrix to form a 'composite'29. Such inulti-layeredcomposites can be produced either in one operation by consolidatingseveral rows of wires between a number of aluminum sheets, or else, bypassing several layers of the composites 23 through ahot rolling mill.

From the foregoing description, one skilled in the art can easilyascertain'the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are properly,equitably, and intended to be, within the full range of equivalence ofthe following claims.

What we claim is:

1. In a process for the production of a metallic composite from anassembled stack of at least two sheets of aluminum having steel wiresandwiched therebetween, wherein one of the opposing sheet surfaces hasa groove formed therein extending transverse to the direction of thewire, the step comprising: compressing the assembled stack at anelevated temperature with a sufficient force to fill in the groove andexpose fresh metal surface which can form a substantially continuousmetallurgical bond between the sheet surfaces.

2. Proces as defined by claim 1, wherein the assembled stack iscompressed at a temperature of from about 400 to 1100 F. for a period offrom about minutes to 10 hours.

3. Process as defined by claim 1, wherein the assembled stack iscompressed at a temperature of from about 700 to 900 F. for a period offrom about /2 to 1 /2 hours.

4. Process as defined by claim 1, wherein at least one of the opposingsheet surfaces has a plurality of elongated grooves extending transverseto the direction of the wire.

5. Process as defined by claim 1, wherein the assembled stack iscompressed by hot rolling.

6. Process as defined by claim 5, wherein the hot rolling is carried outwith the wires extending normal to the axis of the rolls.

7. Process as defined by claim 5, wherein the hot rolling is carried outwith the groove in the sheet surfaces extending parallel to the axis ofthe rolls.

8. Process as defined by claim 1, wherein compression of the assembledstack is carried out in an inert atmosphere.

9. Process as defined by claim 1, wherein compression of the assembledstack is carried out under a vacuum.

References Cited UNITED STATES PATENTS 867,659 10/1907 Hooper 29l96.23,201,862 8/1965 Gotoh 29470.5X 3,314,825 4/1967 Forsyth et al 148l2X3,406,446 10/ 1968 Muldovan 29497.5 3,419,952 l/ 1969 Carlson 29- 472.33,449,820 6/1969 Jones et al. 29-497.5X 3,489,534 l/1970 Levinstein27-471.1X

JOHN F. CAMPBELL, Primary Examiner R. B. LAZARUS, Assistant Examiner US.Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3 551 996Dated January 5, 1971 Inventor) Edwin V. Sumner, Donald Q. Cole andLeRoy W. Dax

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 3, Line 70, change "film can impair to ---film which canimpair--- Column 4, Line 7, change "200 ton" to ---2000 ton- Signed andsealed this 15th day of June 1971.

(SEAL) Attest:

,EDWARD M.FIETCHER,JR. WILLIAM E. SGHUYLER, Attesting OfficerCommissioner of Patem

